Ground wiring gauge

wxh3

In my small standalone system (270W solar, 220AH batteries, 300W inverter) I have 10 gauge wire for grounding my solar panel frames along with equipment (charger controller, inverter (AC & DC)) (Note, this is also the gauge wire I am using for all D/C power connections..and am using 12 gauge for AC.) Ground itself is a 10 foot copper rod buried in ground.

Is this adequate? I've believe I've read somewhere recommending 6 or 8 gauge for at least some of the grounding wires.

mike95490

Re: Ground wiring gauge

Heaver gauge will not hurt. It's supposed to be in an armored sheath, to prevent it from being damaged.

System2

Re: Ground wiring gauge

Hi wxh3,

I am new to this and thought I'd try to research your question for my own sake.

If your system is over 50 volts (125% of Voc), one current carrying conductor (usually the negative) must be grounded. If it is under 50 volts and ungrounded, you need overcurrent protection (breakers) on both current carrying wires.

Roof-mounted panels require ground-fault protection per the NEC.

All exposed metal non-current carrying parts of the system must be grounded independent of system voltage.

From my SEI book on Photovoltaics:
Size of Grounding Electrode Conductor
"The DC system grounding electrode conductor, which is the bare copper wire connecting grounded conductor (the negative wire) and/or equipment grounding conductor to the grounding electrode (the ground rod), cannot be smaller than #6 AWG aluminum or #8 AWG copper or the largest conductor supplied by the system (NEC 250.166). Even though many PV systems have larger conductors in the system (for example #4/0 inverter cables), they can use #6 AWG copper wire for the grounding electrode conductor if that is the only connection to the grounding electrode (NEC 250.166(C))."
Source: Photovoltaics Design and Installation Manual, p. 109
Solar Energy International, Fourth Printing June 2006

But then from "Photovoltaic Power Systems And the 2005 National Electrical Code: Suggested Practices":
"In a few cases, the direct-current system-grounding electrode conductor shall not be smaller than 8 AWG or the largest conductor supplied by the system [250.166(B)]."

This refers to the grounding wire from your equipment to your ground rod. So it seems that you can use #8 if the system doesn't have any larger wires.

The discussion for the ground from the panels to your equipment is a bit longer and depends on use of a ground-fault protection, size of the current carrying conductors and if those conductors are oversize for voltage drop.

From the Suggested Practice book:

"All PV systems, regardless of voltage, must have an equipment-grounding system for exposed metal surfaces (e.g., module frames and inverter cases) [690.43]. The equipment-grounding conductor shall be sized as required by Article 690.45 or 250.122. Generally, this will mean an equipment-grounding conductor (in other than PV source and output circuits) based on the size of the overcurrent device protecting the ac or dc circuit conductors. Table 250.122 in the NEC gives the sizes."

I don't have a copy of the NEC, so I don't have Table 250.122.

"What size conductor should be used? The minimum code requirement is for the equipment grounding conductor for PV source and output circuits to be sized to carry 1.25 times the short-circuit currents at that point. While this may allow a 14 AWG conductor between modules, a conductor this small would require physical protection between the grounding points. Some inspectors will allow a 10 AWG bare conductor to be routed behind the modules from grounding point to grounding point if the conductors are well protected from damage, as they would be in a roof-mounted array. If needed, an 8 AWG or 6 AWG sized conductor may be required (to meet the code or to satisfy the inspector) and then the ILSCO lugs should be used."

It seems that the #10 for the panels is sufficient as long as this would handle 1.25 times Isc of the array. For the connection to the ground rod, it seems to say you need #8. Take care to not have copper and aluminum in direct contact. Keep in mind that I may be all wrong! I'm just trying to take a stab at it. Any other help?

Cheers,
Rob

wxh3

Re: Ground wiring gauge

Thanks. So it seems I am endangering my equipment and/or life by using #10 wire. Guess I will go buy some #8 wire.

wxh3

Re: Ground wiring gauge

I notice my inverter instructions state DC power wires should be 6 AWG or larger and ground 4 AWG or larger (and larger than power wires). I wonder if the the 6 AWG wire is really necessary for a 300W inverter....my cable runs are very short.

niel

Re: Ground wiring gauge

use of #8 will be fine for the grounding, but seeing as you are already going to be using #6 you could use that too. though the #4 would be nice for your ground, i think it to be overkill.

icarus

Re: Ground wiring gauge

wxh3 wrote: »

I notice my inverter instructions state DC power wires should be 6 AWG or larger and ground 4 AWG or larger (and larger than power wires). I wonder if the the 6 AWG wire is really necessary for a 300W inverter....my cable runs are very short.

It is all a matter of voltage drop and what you consider acceptable. (and NEC/CE if you care. Remember 300 watts @ 120 vac is 2.5 amps, but the same 300 watts at 12vdc is ~25 amps. If you use # 10 and fuse it for # ten wire size maximum, (I can't remember, 30 amps?) then you would be safe if you could live with the voltage drop.

On the other hand, voltage drop is all waste that has to be generated somewhere.

Tony

PS Having said all that, I had a 300 watt inverter 120' away from the battery, #10/2uf wire, fused for 15 amps. I only ran a 60 watt load max, and 20 watts most of the time. At 60 watts, I had almost a whole volt drop under load.

niel

Re: Ground wiring gauge

"PS Having said all that, I had a 300 watt inverter 120' away from the battery, #10/2uf wire, fused for 15 amps. I only ran a 60 watt load max, and 20 watts most of the time. At 60 watts, I had almost a whole volt drop under load."
__________________

at a temp of 68 degrees f or 20 c the drop should've been about 1.2v. just a thought here, did you think of inverting at the battery and sending the ac down that 120ft run? the lower current sent from the inverter would lower the vdrop coupled with the fact the higher voltage would lower the percentage for a given vdrop it would've been much better to do. the ac would've seen a vdrop of around .12v. the drawback is having to go to the inverter to turn it on/off or devising another way to power it on/off remotely.

icarus

Re: Ground wiring gauge

Neil, the operative words in that post was "used to have". The said installation was temp. (As for the real voltage drop, the story is a bit more anecdotal, as I don't know the exact wire length, nor the exact drop/load combinations.

T

niel

Re: Ground wiring gauge

yes i know and that's why i asked in the past tense.